The present invention relates to semiconductor device manufacturing, and more particularly to methods of incorporating nitrogen at or near a silicon surface so as to be capable of forming either an epi-silicon-containing layer or a silicide layer on selective portions of the silicon surface.
In the manufacturing of semiconductor devices, it is well known that nitrogen present at or near the silicon surface of a semiconductor device changes many surface properties of the silicon. The ability to introduce nitrogen in different areas of a semiconductor chip or wafer allows for fabrication of different structures and devices in a cost effective manner, by reducing the number of steps required to make the structure.
As stated above, nitrogen affects many surface, or near surface properties of a semiconductor device. Some of the more significant ones include: (i) oxidation rate; (ii) silicide formation; (iii) selective epitaxial growth; and (iv) boron diffusion.
In regard to oxidation rate, it is well known that nitrogen reduces oxidation rate and that a higher concentration of nitrogen formed within or near a silicon surface results in slower oxidation rate. Insofar as silicide formation is concerned, nitrogen serves to block or reduce silicide formation by preventing the reaction between a refractory metal, e.g., Ti or Co, and the underlying silicon layer.
In epitaxial growth, which occurs on bare silicon surfaces without nitrogen at the surface, nitrogen can be used in such a process to prevent the single crystal deposition of epitaxial silicon. That is, nitrogen can be used as a surface masking layer so as to prevent epitaxial silicon growth from occurring in the regions containing nitrogen.
With respect to boron diffusion, nitrogen reduces boron diffusion; therefore nitrogen can be used to prevent channeling of boron into silicon surfaces, i.e., boron diffusion from a gate region into an underlying silicon surface.
Various prior art methods for patterned incorporation of nitrogen into a surface of a silicon-containing substrate are known and have been successfully employed in the semiconductor industry. For example, it is known in the prior art to implant nitrogen into selective regions by utilizing photoresist patterning. Such a prior art process is depicted in FIG. 1 wherein reference numeral 10 denotes a Si-containing substrate, reference numeral 12 denotes a patterned photoresist and reference numeral 14 denotes N+ or N2+ ions being incorporated into Si-containing substrate 10 via ion implantation. The region labeled as 16 in FIG. 1 denotes the area in which nitrogen is implanted within substrate 10. The prior art process depicted in FIG. 1 is advantageous because the ion implantation process is typically carried out at room temperature which is directly compatible with the patterned photoresist.
Despite the above advantages, the prior art nitrogen ion implantation process, which is typically performed at high doses, causes resist hardening. Hardened resist are difficult to remove using conventional stripping processes well known in the art. Additionally, prior art nitrogen ion implantation processes may cause undesirable contamination problems. In particularly, prior art nitrogen ion implantation processes may result in implant damage in the substrate which may lead to increased oxidation rate and degraded mobility of implanted ions within the substrate. Moreover, in the prior art, nitrogen is typically implanted well below the surface of the silicon, not at the surface. Hence, a thermal cycle is needed to diffuse the implanted nitrogen ions to the silicon surface and residual nitrogen may be left in the substrate where it can have undesirable effects.
Another prior art method of introducing nitrogen into a surface is by utilizing a thermal nitridation process. Typically, prior art thermal nitridation processes are performed at temperatures greater than about 700xc2x0 C. using NO, N2O or NH3 as the nitrogen-containing ambient. A typical prior art nitridation process is shown, for example, in FIGS. 2A-D. Specifically, FIG. 2A comprises Si-containing substrate 10, hard masking layer, e.g., an oxide or nitride, 18, and patterned photoresist 12. Such a structure is fabricated using conventional techniques well known in the art. Since the photoresist cannot withstand the high-temperature nitridation process, it is employed in the prior art as a means for 10 patterning the underlying hardmask, which can withstand the high-temperatures associated with prior art nitridation process. FIG. 2B shows the structure that is formed after a conventional dry etching process such as reactive ion etching is employed in transferring the pattern from the photoresist to the hardmask and after the resist has been removed.
FIG. 2C shows the resultant structure formed during a typically prior art thermal nitridation process. Note that nitrogen-containing layer 20 is formed during the thermal nitridation process. FIG. 2D shows the structure that is formed after hardmask 18 has been removed from the structure.
One advantage of prior art thermal nitridation processes is that they are capable of implanting nitrogen at or near the surface of a silicon-containing substrate. Disadvantages of prior art nitridation process, however, include: (i) not directly compatible with resist processing because the resist cannot withstand high-temperatures associated with prior art nitridation processes; (ii) to achieve selective nitridation other sacrificial materials such as the hardmask illustrated in FIGS. 2A-2D must be employed (this causes the need for utilizing extra etching, deposition and stripping processes); and (iii) nitrogen may leak through the various layers, e.g., hardmask 18, used to block the silicon surface contaminating areas which are meant to be nitrogen-free.
Another prior art method which can be used in forming a patterned nitrogen-containing region at or near a surface of silicon is described, for example, in U.S. Pat. No. 6,110,842 to Okuno, et al. Specifically, Okuno, et al. disclose a method for forming integrated circuits having multiple gate oxide thicknesses utilizing a high-density plasma nitridation process to reduce the effective gate dielectric thickness in selective areas only. In one embodiment disclosed in Okuno, et al., a patterned mask is formed over a substrate and a high-density plasma nitridation process (low-temperature process) is used in forming a thin layer of nitride or oxynitride on the surface of the substrate. The patterned nitride is thereafter removed and an oxidation is performed. The thin nitride or oxynitride layer retards oxidation, whereas in the areas in which the nitride and oxynitride layer is not present, oxidation is not retarded.
Okuno, et al. disclose a second embodiment in which a thermal oxide is first grown on a substrate. Next, a pattern is then formed that exposes areas where a thinner effective gate oxide is desired. A high-density plasma nitridation is performed converting a portion of the gate oxide to a nitride or oxynitride; the effective thickness of the combined dielectric is reduced.
Although the Okuno, et al. disclosure provides a low-temperature patterned nitridation process for forming a gate dielectric having dual thicknesses, Okuno, et al. do not teach or suggest that the disclosed methods can be used during selective epi or silicide formation. Hence, there is still a need for providing a method that is capable of forming a selective epi or silicide layer on predetermined portions of a silicon-containing substrate using a masked nitridation process that overcomes the drawbacks mentioned in regard to prior art nitrogen ion implantation and thermal nitridation.
One object of the present invention is to provide a selective method of forming a nitrogen-containing layer at or very near the surface of a Si-containing substrate so as to provide a layer that prevents the subsequent formation of an epi-Si-containing layer or a silicide layer.
Another object of the present invention is to provide a low-temperature patterned nitridation (or oxynitridation) process that is compatible with a patterned photoresist that is present on a Si-containing substrate during the low-temperature nitridation (or oxynitridation) process.
A yet further object of the present invention is to provide a method of forming a nitrogen-containing layer at or very near the surface of a Si-containing substrate such that the nitrogen-containing layer formed does not adversely effect either a patterned photoresist previously formed on the Si-containing substrate or the Si-containing substrate itself.
These and other objects and advantages are achieved in the present invention by utilizing a patterned, low-temperature (on the order of about 200xc2x0 C. or below) nitridation process (which also includes oxynitridation) wherein a technique such as decoupled plasma nitridation (DPN), slot plane antenna (SPA) or jet vapor nitridation (JVN) is employed. The low-temperature nitridation process of the present invention is capable of forming a nitride (or oxynitride) layer in areas of a Si-containing substrate (at or near the surface thereof) that are not protected by a patterned resist. The patterned resist is not adversely affected by the inventive method since low-temperatures are employed. Moreover, the nitride (or oxynitride) layer formed in the present invention effectively blocks the regions containing the same so as to be able to form selective epi-Si-containing or silicide layers adjacent to the nitride (or oxyitride) layer.
Oxidation of silicon to form silicon dioxide (SiO2) occurs when silicon is exposed to an oxidizing ambient (O2, H2O) at an elevated temperature (typically above 700xc2x0 C.). The oxidizing species diffuse through the previously grown oxide to the interface where it reacts with the silicon to grow more oxide. Nitrogen incorporated into oxide retards the diffusion of the oxidizing species and hence reduces the oxide growth rate. Silicidation occurs when a refractory metal such as cobalt or titanium is deposited directly on silicon, and heated above 500xc2x0-700xc2x0 C., to form silicides such as CoSi2 or TiSi2. Nitrogen in silicon blocks this reaction by bonding with the silicon, and nitrogen rich areas remain free of silicide formation.
Selective epitaxial growth occurs when a silicon or germanium containing gas such as dichlorosilane is introduced at an elevated temperature. Any single crystalline Si or SiGe areas on the surface of the wafer that are exposed will be deposited with silicon or SiGe. Nitrogen that is incorporated into silicon or SiGe disrupts the crystallinity, and hence there is no epitaxial growth in areas that contain nitrogen.
Specifically, the method of the present invention comprises the steps of:
(a) subjecting at least one exposed surface of a Si-containing substrate to a low-temperature nitridation process so as to form a nitrogen-containing layer at or near said at least one exposed surface, wherein other surfaces of said Si-containing substrate are protected by a patterned photoresist;
(b) removing said patterned photoresist from said other surfaces of said Si-containing substrate; and
(c) forming an epi-Si-containing layer or a silicide layer on said other surfaces of said Si-containing substrate which do not contain said nitrogen-containing layer, wherein said epi-Si-containing layer or said silicide layer is not formed in areas containing said nitrogen-containing layer.
The term xe2x80x9cSi-containing substratexe2x80x9d as used herein denotes a semiconductor material such as Si, SiGe, SiC or SiGeC; layered semiconductors such as Si/Si or Si/SiGe; silicon-on-insulators; polysilicon; amorphous Si; a previously preformed epi-Si layer; or any combination thereof. Thus, the Si-containing substrate may be a semiconductor chip or wafer, or a silicon-containing layer formed atop another material layer, i.e., a semiconductor chip or wafer, a conductive material, or a gate dielectric. The term xe2x80x9cepi-Si-containingxe2x80x9d as used herein denotes epi Si, epi SiGe or a combination thereof.
In accordance with the present invention, the low-temperature nitridation process includes techniques such as DPN, JVN, SPA, or any process that is capable of operating at a temperature of about 200xc2x0 C. or below. Note that the term xe2x80x9cnitridationxe2x80x9d includes nitridation, oxynitridation or any combination thereof.